1. Field of the Invention
This invention pertains to blow-out control fluids and drilling muds which are environmentally compatible and can achieve densities up to about forty (40) pounds per gallon (ppg) while retaining such properties necessary to provide adequate pumpability.
2. Description of the Prior Art
During the drilling of deep oil and gas wells, over-pressured (or geopressured) zones are occasionally penetrated. Since the pressure gradient of these zones seldom exceeds the hydrostatic pressure of a 2.4 density (20 ppg) heavy drilling fluid is often sufficient to control these over-pressured zones. However, such heavy drilling fluids are expensive and are often not effective where the exposed zones have a low parting pressure.
Many problems are incurred when drilling into over-pressurized zones with mud too light to hold back the gas pressure. Such resulting conditions range from gasified mud (for low permeability formations), controllable kicks which are successfully stopped by closing blow-out preventors, to uncontrollable blow-outs. To counteract the over-pressurized zones, high density fluid is pumped into the annulus (backside), drill pipe or casing (if no drill pipe is in hole). If successful, hydrostatic fluid pressure control will be established with respect to the over-pressurized zone. In the case of a blow-out, the surface or intermediate casing is often parted exposing the borehole to any proximate fresh water aquifiers. Additionally, during kill operations, there is often a danger the casing will part before control is established. Furthermore, kill operations for blow-outs are not always successful and kill fluids can be blown from the well and sprayed across the countryside. Hence, any toxic chemicals included in the high density fluid could be transmitted through the aquifier, or directly through the blow-out, to the local animal and plant population.
A variety of drilling fluids and weighting agents are presently on the market. Generally, drilling fluids have an aqueous or hydrocarbon base. One principal requirement of a good drilling fluid is that it is able to suspend a sufficient amount of weighting additives so as to meet desired density requirements particularily with respect to preventing gasification and blow-outs, while remaining pumpable. With respect to aqueous base drilling fluids, a variety of water thickeners are also known. Examples are organic materials such as xanthan gums, aluminum containing compositions, such as hydrous aluminum oxide, polyacrylates, polyacrylamides and a variety of cellulose derivatives. Examples of known weighting materials include barite, hematite, calcium carbonate and bentonite. However, combinations of the above result in weighted drilling fluids having a density of generally less than twenty (20) pounds per gallon.
Under certain conditions conventional mud systems can be weighted up to thirty (30) pounds per gallon using galena as the weighting agent. Other carrier fluids such as zinc bromide and calcium bromide can also be used to carry galena. Lead powders have also been used to increase the density of zinc bromide carrier fluids. However, when wells are in communication with an aquifier the use of any metal or soluble material considered toxic is unacceptable. Additionally, the heavy carrier fluid would not be permitted to contain soluble, transmittable bromide and zinc.
With respect to blow-out control fluids, two desired qualities are good pumpability and a sufficiently high density to equilibrate downhole pressures. In blow-out situations environmental considerations receive additional attention since there is a greater likelihood of communication with an aquifier as well as the possibility of expulsion of fluids during blow-out. The density of fluid necessary for equilibration purposes is also dependent upon the well parameters.
In the art of concrete mixing, particle size interrelationships of the cement, fine sand and coarse aggregate with regard rheology and pumpability properties are a consideration. By adjusting the sand and coarse aggregate particle size distribution, a reduction in the amount of water and cement per unit volume required for good pumpability properties can be reduced. Part of this art is referred to as gap grading. However, it was not previously known whether such principles could be applied to systems where all particles employed are generally smaller than the smallest particle of fine sand in concrete. It was generally anticipated that a point of diminishing returns was already being met with respect to the fine sand particle size.
Therefore, it is a feature of the present invention to provide an environmentally compatible high density fluid to be used in subterranean drilling or blow-out control.